1. Field of the Invention
This invention relates to an optical scanning device used in a color image forming apparatus such as an electrophotographic color copying machine and a color printer and an image forming apparatus for forming an image by the optical scanning device.
2. Description of the Related Art
As a color image forming apparatus such as a color copying machine and a color printer, widely known is the so-called tandem color image forming apparatus in which plural photoreceptors are arranged side by side in a designated direction, toner images of different colors are respectively formed on the plural photoreceptors, and the toner images are sequentially transferred on a transfer medium such as recording paper and an intermediate transfer belt to form a color image.
FIG. 9 is a schematic diagram of the conventional tandem color image forming apparatus.
As shown in FIG. 9, the color image forming apparatus includes photoreceptor drums 101a, 101b, 101c, 101d which are arranged in parallel in the transport direction B of recording paper 106 and on the surfaces of which electrostatic latent images of Y (yellow), M (magenta), C (cyan) and BK (black) colors are respectively formed, corotron chargers 102a, 102b, 102c, 102d for uniformly charging the respective photoreceptor drum surfaces, optical scanning devices 103a, 103b, 103c, 103d for emitting a laser beam modulated according to each image piece of Y, M, C, BK obtained by conducting designated processing for the color data of R (red), G (green), B (blue) to the respective photoreceptor drum surfaces, cylindrical mirrors 104a, 104b, 104c, 104d for respectively changing laser beams emitted from the optical scanning devices to an optical path heading toward the corresponding photoreceptor drum, developing devices 105a, 105b, 105c, 105d for developing the electrostatic latent images formed on the respective photoreceptor drum surfaces by irradiation of a laser beam with toner of Y, M, C, BK to form toner images, a transport belt 107 for transporting the recording paper 106 in the transport direction B along the tangential line common to the photoreceptor drums to transfer the toner images of the respective colors formed on the photoreceptors onto the recording paper 106, cleaners 108a, 108b, 108c 108d for removing toner remaining on the photoreceptor drums, and a fixing roll 109 for fixing the transferred images on the recording paper 106 transferred from the photoreceptors.
FIG. 10 is a schematic diagram of an optical scanning device used in the conventional color image forming apparatus.
FIG. 10 illustrates the optical scanning device 103a for Y color among four optical scanning devices used in the color image forming apparatus shown in FIG. 9. The other three optical scanning devices 103b, 103c, 103d have the substantially same constitution as the optical scanning device 103a, so the description will be omitted.
The optical scanning device 103a includes a light emission time control circuit 110 for controlling the light emission time of a laser beam according to the image data of Y color, a laser diode 111 controlled by the light emission time control circuit 110 to emit a modulated laser beam, a collimator lens 112 for converging a laser beam emitted from the laser diode 111, a cylindrical lens 113 for converging a laser beam converged by the collimator lens 112 to diffuse from a focal point, a polygon mirror 114 for deflecting a laser beam passed through the cylindrical lens 113, an fxcex8 lens 115 for focusing a light beam deflected by the polygon mirror 114 in the fast-scanning direction to scan on the exposure line of the photoreceptor drum 101a at uniform speed, and a cylindrical mirror 104a for finally imaging a laser beam on the photoreceptor drum 101a. 
The cylindrical mirror 104a used is so constructed that one face of a glass member like a rectangular parallelepiped is polished to be recessed like a cylindrical surface, and metal having high reflectance such as aluminum is deposited, and in some case, coating for further heightening the reflectance is conducted.
The operation of the color image forming apparatus shown in FIG. 9 will now be described. The surfaces of the photoreceptor drums 101a, 101b, 101c, 101d rotated in the direction of an arrow A are previously uniformly charged by the chargers 102a, 102b, 102c, 102d, and laser beams modulated according to the respective image data of Y, M, C, BK are emitted from the optical scanning devices 103a, 103b, 103c, 103d to the charged surfaces to form the electrostatic latent images on the surfaces of the photoreceptor drums. The electrostatic latent images are developed with toner of the respective colors by the developing devices 105a, 105b, 105c, 105d to form toner images on the photoreceptor drum surfaces. The toner images are supported on the photoreceptor drums to be transported to a transfer position where the photoreceptor drums come into contact with the recording paper 106. On the other hand, the recording paper 106 placed on the transport belt 107 is supported on the transport belt 107 to be fed to the transfer position of each photoreceptor drum in the same timing, whereby toner images of the respective colors on the photoreceptor drums are sequentially transferred onto the recording paper 106. The recording paper 106 to which the toner images are transferred is transported to the fixing roll 109, and the transferred images are fixed to obtain a color image.
The conventional tandem color image forming apparatus, however, has the problem that as the plural photoreceptor drums are exposed by the plural optical scanning devices corresponding thereto, the apparatus is increased in size leading to a cost increase. Further, a problem exists that as the optical elements constituting each optical scanning device are independently varied due to thermal deformation of an enclosure, the image formation position on the scanned surface of the photoreceptor drum is varied so that the image forming positions of the respective colors on the recording paper are shifted to remarkably lower the image quality.
Recently, as disclosed in Japanese Published Unexamined Patent Application No. Hei 10-20608, an optical scanning device has been proposed to reduce color aberration while reducing the size and the cost by making the optical parts related to plural laser beams for exposing the plural photoreceptor drums in common.
FIG. 11 is a schematic diagram of the conventional optical scanning device in which the optical parts are made in common.
As shown in FIG. 11, the optical scanning device is formed by a semiconductor laser array 125 or emitting four laser beams modulated according to each image data of Y (yellow), M (magenta), C (cyan) and BK (black), a polygon mirror 129 for commonly deflecting four laser beams emitted from the semiconductor laser array 125, an fxcex8 lens 130 for focusing four light beams deflected by the polygon mirror 129 in the fast-scanning direction to scan on the exposure lines of the photoreceptor drums 124a, 124b, 124c, 124d at uniform speed, a separating polygon mirror 131 which is formed by combination of four mirror surfaces having four planes of incidence different in angle for the four laser beams passed through the fxcex8 lens 130 and adapted to separate four laser beams from each other in the directions corresponding to the array positions of the photoreceptor drums, and cylindrical mirrors 132a, 132b, 132c, 132d for focusing for laser beams separated by the separating polygon mirror 131 in the slow-scanning direction to be imaged on the corresponding photoreceptor drums.
In the optical scanning device thus constructed, four laser beams modulated according to the image data of Y, M, C, BK are emitted from the semiconductor laser array 125, commonly deflected by the polygon mirror 129 to enter the separating polygon mirror 131 through the fxcex8 lens 130, and separated in the directions corresponding to the array positions of the photoreceptor drums 124a, 124b, 124c, 124d by the separating polygon mirror 131. The separated four beams are respectively reflected by the cylindrical mirrors 132a, 132b, 132c, 132d and guided to the scanned surfaces 123a, 123b, 123b, 123d of the corresponding photoreceptor drums to expose the scanned surfaces 123a, 123b, 123c, 123d of the photoreceptor drums, thereby forming electrostatic latent images on the photoreceptor drums.
In the optical scanning device, however, though the optical system from the semiconductor laser array 125 to the separating polygon mirror 131 is used in common so as to hold down color aberration due to the change with the passage of time of the respective optical elements to the minimum, four cylindrical mirrors 132a, 132b, 132c, 132d are separately supported to easily cause independent change with the passage of time. Further, the cylindrical mirrors and the photoreceptor drums do not have optically conjugate relationship. Therefore, when the angle of the cylindrical mirror changes by xcex94xcex1, the advancing direction of light headed from the cylindrical mirror to the photoreceptor drum is changed by 2xcex94xcex1 so that the image formation position on the photoreceptor drum is largely varied to cause great color aberration.
As the actual change with the passage of time of the cylindrical mirror, the change of angle and the change of position are considered. Among them, the major problem is the angle change, and among the angle changes taking the three directions of the fast-scanning direction, the slow-scanning direction and the direction of the optical axis as the axis of rotation, it is the angle change taking the fast-scanning direction as the axis of rotation that has a greater influence on the occurrence of color aberration. The reason is that in general thermal deformation, the difference in the position change between both ends of the cylindrical mirror is small, and since the cylindrical mirror is as comparatively long as about 190 mm long even with the A4 machine, the angle changes taking the slow-scanning direction and the direction of the optical axis as the axis of rotation are not so large.
The present invention has been made in view of the above circumstances and provides an optical scanning device for color image formation in which color aberration due to a change of the image formation position on a scanned surface caused by the angle change of the optical system is restrained.
The optical scanning device of the present invention includes a light beam emitting part for emitting plural light beams, a deflecting part for commonly deflecting the plural light beams emitted from the light beam emitting part, a separating optical system for separating the plural light beams deflected by the deflecting part from each other, and beam corresponding optical systems respectively corresponding to the plural light beams for guiding the plural light beams separated by the separating optical system to respective designated scanned surface, wherein each of the beam corresponding optical systems respectively includes a reflecting member having two mirror surfaces arranged like a V-shape for reflecting the corresponding light beam, one time each, among the plural light beams separated by the separating optical system.
In this arrangement, preferably one mirror surface of the two mirror surfaces is a concave cylindrical mirror surface, and the other mirror surface is a plane mirror surface.
Further, a preferable mode is that the cylindrical mirror surface is arranged on the downstream side of the optical path of a light beam from the plane mirror surface.
The optical scanning device of the present invention includes a light beam emitting unit for emitting plural light beams, a light beam deflecting unit for commonly deflecting the plural light beams emitted from the light beam emitting unit, a separating optical system for separating the plural light beams deflected in the light beam deflecting unit from each other, and beam corresponding optical systems respectively corresponding to the plural light beams, for guiding the plural light beams separated by the separating optical system to respective designated scanned surface, wherein the beam corresponding optical systems respectively have two mirror surfaces for reflecting the corresponding light beam, one time each, among the plural light beams separated by the separating optical system.
In this case, preferably the separating optical system is formed by a single separating optical element for separating the plural light beams from each other.
The two mirror surfaces may be fixed to a support member common to the two mirror surfaces, or the two mirror surfaces may be individually fixed to separate support members.
Preferably, the two mirror surfaces constituting the beam corresponding optical system are arranged on the same side of a space halved by the plane formed by the optical path of a light beam deflected at the light beam deflecting unit in each one beam corresponding optical system.
Preferably, the beam corresponding optical systems are respectively adapted to guide light beams separated by the separating optical system to the corresponding scanned surface through an optical path free from intersecting a plane formed by an optical path for a light beam deflected at the light beam deflecting unit.
Further, preferably, the separating optical system and the beam corresponding optical system are adapted to keep each optical path length of the plural light beams between each deflecting point at the light beam deflecting unit and the irradiated point on each scanned surface equal to each other.
The two mirror surfaces may be formed by the mirror surfaces of separate mirrors, or two mirror surfaces constituting one prism.
According to the present invention, the image forming apparatus includes plural photoreceptors where an electrostatic latent image is recorded by light beams modulated according to the image data, and is adapted to develop plural electrostatic latent images formed on the plural photoreceptors with toner of the respective colors to form toner images of the respective colors, and finally transfer and fix the respective toner images on a designated recording medium to record the images on a recording medium, wherein the image forming apparatus includes an optical scanning device having a light beam emitting unit for emitting plural light beams respectively modulated according to data of each image, a light beam deflecting unit for commonly deflecting the plural light beams emitted from the light beam emitting unit, a separating optical system for separating the plural light beams deflected by the light beam deflecting unit from each other, and beam corresponding optical systems respectively corresponding to the plural light beams for guiding the plural light beams separated by the separating optical system to the plural photoreceptors, respectively, the beam corresponding optical systems being respectively provided with two mirror surfaces for reflecting the corresponding light beam, one time each, among the plural light beams separated by the separating optical system.